1. Field of the Invention
This invention relates to a switching power supply which supplies a stabilized d-c d-c voltage to industrial or commercial electronic apparatus.
2. Description of the Prior Art
According to a demand or electronic apparatus with smaller size, higher performance, less power consumption, and lower cost, switching power supplies with smaller size, higher stability of d-c output voltage, higher efficiency, and lower cost have been strongly requested.
Two types of switching power supplies are explained below.
FORWARD type
FIG. 1 is a circuit diagram of a "Forward type" switching power supply in accordance with the prior art. In FIG. 1, 1 is a d-c source which is usually obtained by filtering after rectifying of commercial a-c voltage or a battery. The terminals 2 and 2' of the d-c source 1 are a positive side and a negative side, respectively. 3 is a transformer having a primary winding 3a, a secondary winding 3b, and a reset winding 3c. A terminal of the primary winding 3a is connected to the positive side 2 of the d-c source 1 and another terminal is connected to the negative side 2' of the d-c source 1 via a switching device 4.
A terminal of the secondary winding 3b is connected to the positive terminal 13 of the d-c output via a rectifying diode 15 and aninductor 17 and another terminal of the secondary winding 3b is connected to the negative terminal 13' of the d-c output.
A terminal of the reset winding 3c is connected to the positive terminal of the d-c source 1 and another terminal is connected to the negative terminal of the d-c source 1 via a diode 18. The switching device 4 is switched by a signal from a control circuit 14 and applies and cuts the d-c input voltage to the primary winding 3a of the transformer 3. The diode 15 leads an induced voltage in the secondary winding 3b when the switching device 4 is on to the inductance 17.
A clamping diode 16 is on when the diode 15 is off and makes the negative side of the voltage induced in the secondary winding 3b zero volt. The inductance 17 supplies d-c current to the output terminal 13-13' by filtering the voltages through the diodes 15 and 16. A filtering capacitor 11 is connected between the output terminals 13 and 13' and holds the voltage averaged by the inductor 17 and itself.
The control circuit 14 detects the voltage across the output terminals 13 and 13' and changes an on/off duty ratio of the switching device 4 so that the output voltage is held constant. The diode 18 clamps flyback pulse induced in the reset winding 3c when the switching device 4 turns to off, resets a magnetic flux of the transformer 3, and absorbs a spike voltage.
FIG. 2(a) to (d) illustrate the waveforms in the switching power switching device 4, (b) is a current waveform Id which flows in the switching device 4, (c) is an on/off signal Vg of the control circuit 14, and (d) is the voltage waveform Vs which is applied to a terminal of the inductance 17.
When the switching device 4 turns to on at the time t1 by the on/off signal Vg, a spike current flows in the switching device 4.
This is due to a charging and discharging current to a distributed capacitance such as interwinding capacitances and an interlayer capacitance and a discharging current of a parastic capacitance of the switching device 4. This spike current induces increased noise, decreased reliability and increased power loss.
When the switching device 4 turns on and Vds becomes small enough, the input voltage Vin is applied to the primary winding 3a of the transformer 3 and a voltage (Vin/n) is induced at the secondary winding 3b and the diode 15 becomes on. Where, n is a turn-ratio of the primary winding 3a to the secondary winding 3b. When the diode 15 becomes on, the diode 16 becomes off and the voltage across the diode 16 Vs becomes (Vin/n) and the current in the inductor 17 flows into a load.
Therefore, in the primary winding 3a, a sum of the primary current (Io/n) converted from the current Io in the secondary winding 3b and an exciting current of the primary winding 3a. When the switching device 4 turns to off at the time t2 by the on/off signal Vg, a spike voltage is induced in the primary winding 3a due to a leakage inductance. This spike voltage becomes noise and power loss.
When a flyback voltage is induced in the reset winding 3c and the diode 18 becomes on, the input d-c source is applied across the reset winding 3c. As the turn-ratio of the reset winding 3c to the primary winding 3a is made as to (1 to 1), a voltage of (2.times.Vin) appears across the switching device 4, and in the secondary winding 3b a voltage which gives reverse bias to the diode 15 is induced. Therefore, the diode 15 becomes off and the diode 16 becomes on by the current of the inductor 17. Consequently, the voltage across the diode 16 becomes 0. At the same time, a recovery voltage appears across the diode 15 and current and voltage ringings are generated which causes noise and power loss. When the current of the reset winding 3c becomes 0 at the time t3, the diode 18 becomes off and the voltage across the primary winding 3a becomes 0 and the d-c source voltage Vin is applied across the switching device 4.
As no voltage is induced in he secondary winding 3b at this time, the diode 15 holds off state and the diode 16 holds on state. When the switching device 4 turns to on by the on/off signal Vg from the control circuit 14, the d-c source voltage Vin is applied across the primary winding 3a and then the voltage (Vin/n) appears across the secondary winding 3b, the diode 15 turns to on, and the diode 16 turns to off. At this time, a recovery voltage is generated also in the diode 16 and current and voltage ringings are generated and they make noise and power loss.
As the output voltage Vout is a mean value of the voltage Vs, EQU Vout=[Ton/(Ton+Toff)].times.(Vin/n),
where Ton and Toff is an on-period and an off-period of the switching device 4, respectively.
Thus, regulation of the output voltage is possible by changing the on/off ratio of the switching device 4.
FLYBACK type
FIG. 3 is a circuit diagram of a "Flyback type" switching power supply in accordance with the prior art. In FIG. 3, the components which have the same functions as those in FIG. 1 have the same number and their descriptions are omitted.
FIG. 4 (a) to (d) illustrate waveforms in the switching power supply shown in FIG. 3.
(a) is a voltage waveform Vds across the switching device 4, PA1 (b) is a current waveform Id flowing in the switching device 4, PA1 (c) is an on/off signal Vg from the control circuit 14, which controls the switching device 4, and PA1 (d) is a voltage waveform Vs which is induced across the secondary winding 3b.
When the switching device 4 turns to on at the time t1 by the on/off signal Vg, a spike current flows in the switching device 4. This is due to a charging and discharging current to a distributed capacitance such as interwinding capacitance and interlayer capacitance and due to a discharging current of a parastic capacitance which relates to the switching device 4. This spike current induces increased noise, decreased reliability and increased power loss.
When the switching device 4 is on, the voltage across the switching device 4 Vds is small enough and the input voltage Vin is applied to the primary winding 3a of the transformer 3. And a voltage (Vin/n) is induced across the secondary winding 3b and the diode 10 is biased inversely and becomes off. Consequently, the exciting current of the transformer 3 flows in the primary winding 3a increasingly. Here, n is a turn-ratio of the primary winding 3a to the secondary winding 3b .
When the switching device 4 turns to off by the on/off signal Vg at the time t2, a spike voltage due to a leakage inductance of the transformer 3 is induced. This spike voltage causes noise and power loss.
And a flyback pulse is induced across the secondary winding 3b and the diode 10 becomes on and is clamped at the output voltage Vout which is held in the capacitor 11. The voltage (Vin+n.times.Vout) is applied across the switching device 4. When the switching device 4 turns to on by the on/off signal at the time t3, the d-c source voltage Vin is applied across the primary winding 3a. The voltage (Vin/n) is induced across the secondary winding 3b and the diode 15 turns to off.
The output voltage Vout is expressed as follows. EQU Vout=(Ton/Toff ).times.(Vin/n),
where Ton is an on-period and Toff is an off-period of the switching device 4. Thus, a regulation of the output voltage is possible by changing an on/off ratio of the switching device 4.
However, in the prior art, both in a forward type and in a flyback type, a spike current is induced when the switching device 4 turns to on, a spike voltage is induced when the switching device 4 turns to off, and a recovery is generated when the diode 16 or 10 turns to off. This is a problem because the spike current, the spike voltage, and a diode recovery make noise, and give some interference to electronic apparatus and make some power losses.